Unfortunately, it is not possible to list items under more than one sub-category. We understand that at times, it may be possible that items could fall under multiple sub-categories. However, the purpose of subcategories is to help streamline searches and the best way to achieve this would be for items to be listed in only one sub-category.
In cases whereby you think your item can be categorised in more than one sub-category, we would advise you to choose the one sub-category which best fits the item and list it under that sub-category. We also have an example chart for Services listings on Carousell.
Not to worry though, we're always looking to improve on our categorisation model depending on market demand and popularity!